A Syrian air raid killed 30 people in the rebel-held northern border town of Azaz on Wednesday, a local doctor said, and a bomb went off near UN and military sites in the capital Damascus, wounding three.

The doctor, Mohammad Lakhini, said at a hospital in Azaz that scores of people were wounded in the air strike. It reduced several houses in the town to rubble and dozens of men clawed through the concrete and metal debris looking for survivors.

As the violence intensified, UN human rights investigators accused forces loyal to Syrian President Bashar Assad of committing war crimes and crimes against humanity.

They said rebels had also committed war crimes, but the violations "did not reach the gravity, frequency and scale" of those by state forces and the pro-Assad shabbiha militia.

"The commission found reasonable grounds to believe that government forces and the shabbiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law, including unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property," said the 102-page report by the independent investigators led by Paulo Pinheiro.

In s video posted by activists earlier on Wednesday, residents in Azaz - situated just north of the major urban battleground of Aleppo - screamed and shouted "God is greatest" as they carried bloodied bodies from collapsed concrete buildings.

The opposition Syrian Observatory for Human Rights said dozens had been killed. One activist in the town said at least 30 bodies had been found and rescuers were searching for more.

The video footage, which could not be immediately verified, showed crowds of residents wrestling with steel bars and pulling away a giant slab of concrete to reveal the dust-covered arm of a child. "This is a real catastrophe," said an activist who gave his name only as Anwar. "An entire street was destroyed."

Seven Lebanese hostages being held in Azaz were also wounded, with four others still missing, a rebel commander said.

"The building they (the hostages) were in was hit ... We were able to remove seven from the wreckage. They are wounded, and some of the injuries are serious," rebel commander Ahmed Ghazali told the Lebanese news channel Al Jadeed.

Assad's forces have increasingly used helicopter gunships and warplanes against the lightly-armed insurgents.

In Damascus, a bomb exploded in the car park of a hotel used by UN monitors, but several military buildings are also in the vicinity and it was not clear what the target was.

No UN staff were hurt in the blast, which occurred exactly four weeks after a bomb killed four of Assad's senior aides. The bomb set a fuel tanker ablaze and black smoke billowed over the city. Ash and dust covered white UN vehicles parked nearby.

State media said three people were wounded in the bombing and several rebels were killed or captured in a separate gunbattle with security forces in the western district of Mezze.

Deputy Foreign Minister Faisal Mekdad said the bomb blast proved "the criminal and barbaric nature of those who carry out these attacks - and their backers in Syria and abroad."

UN emergency relief coordinator Valerie Amos, on a mission to seek more access for aid deliveries, was meeting European Union officials in Damascus when the bomb went off.

She herself was unable to reach the town of Douma, a trouble spot just north of the capital, due to bombardment.

"Waiting at checkpoint to get into Duma. Sounds of shelling. Could not enter," Amos tweeted. The authorities told her she had been turned back for her own safety, her spokesman said later.

Syrian state media said Amos met Foreign Minister Walid al-Moualem to discuss the growing needs of civilians affected by the "destruction of private and state property by terrorist armed groups" - the government's usual term for rebels.

Groups calling themselves The Descendants of the Prophet Brigade and the al-Habib al-Mustafa Brigade said on a Facebook page they were jointly responsible for the bombing and that the attack had killed 50 soldiers. It was impossible to verify that claim.

Last month Assad's troops successfully counter-attacked after rebels seized parts of Damascus. They are still trying to dislodge insurgents from Aleppo, Syria's biggest city.

More than 50 were killed across Syria on Wednesday, the Syrian Observatory said, after 160 the day before.

Opposition sources say 18,000 people have been killed since the uprising against Assad erupted in March last year.

The bloodshed has divided regional and world powers, foiling peace efforts and paralysing the UN Security Council on Syria.

Russia accused the West on Wednesday of reneging on an agreement to establish a transitional government in Syria and of prolonging the bloodshed by encouraging the rebels to keep fighting to overthrow Assad.

Foreign Minister Sergei Lavrov said an agreement made by world powers and the then-peace envoy Kofi Annan in Geneva on June 30 was still valid and urged the West to do more to put it into practice.

Most Western and Arab governments have called on Assad to go, saying his government's violent response to initially peaceful protests give him no place in a future Syria.

Russia has opposed tougher UN sanctions against Damascus, a long-time strategic ally, but denies it is actively helping Assad remain in power.

Muslim heads of state were expected to suspend Syria from the Organisation of Islamic Cooperation at a summit in Mecca on Wednesday, over the objections of Iran, Assad's closest ally.

The 57-member body's rebuke is mostly symbolic, but it shows Syria's isolation in much of the Sunni-majority Islamic world.

Syria's own Sunni majority is the dynamo of the revolt against Assad, whose Shi'ite-rooted Alawite minority is at the core of a ruling system based on the army and security services.

Syria has been caught up in a wider sectarian-tinged tussle pitting Shi'ite Iran against Saudi Arabia and its Sunni-ruled Gulf allies. Turkey has also turned against Assad. Tehran is determined to prop up a proven ally who has provided vital support for its Shi'ite ally Hezbollah in Lebanon.

Ankara and Washington have pledged to step up aid to the Syrian opposition and planning for a post-Assad Syria.

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